Preparation of nitrocelluloses



Patented Sept. 27, 1932 MILTON O. SCH'UR AND BENJAMIN G. HOOS, OFBERLIN, NEW HAMPSHIRE, ASSIGNORS TO BROWN COMPANY, OF BERLIN, NEWHAMPSHIRE, A OORPORATION OF MAINE PRERARATION OF No Drawing. Applicationfiled September This invention relates to the preparation ofnitrocelluloses using cellulose fibers of various kinds and in variousphysical forms as the raw material. It devolves more specifically aboutthe drowning treatment with Water to which the freshly nitratedcellulose fiber is subjected while still wet with a large excess of thestrong mixed nitrating acids carried out of the nitrating bath. In thepreparation of nitrocelluloses, th current practice is to steepcellulose fiber in suitable physical form in a bath of mixed nit-ratingacids containing far more acid than that which is actually consumed bythe reaction upon the cellulose fiber. After the fiber has been nitratedto the desired-nitrogen content, the nitrocellulose is separated fromthe bath of mixed nitrating acids and the acid associated therewith israpidly diluted with large quantities of water at seasonabletemperatures. This step is technically known as the drowning operation,because the nitrocellulose is submersed completely in large quantitiesof diluting water. There is good reason for this sort of practice, for,in order to avoid hydrolysis and accompanying loss of thenitrocellulose, the waterused for removing essentially all of themixednitrating acids must practically instantaneously .permeate theentire nitrated charge. Otherwise, local over-heating may ensue, onaccount of theheat of dilution of the mixed nitrating acids. The largeexcess of water serves as a reservoir for absorbing the heat, whereforethe temperature of the drowned batch of nitrocellulose is notappreciably above that of the diluting water. For example, after a batchof cotton linters has been nitrated in the usual nitrating pot and hasbeen centrifuged or wrung free of easily removable acid, the batch isdischarged from the centrifuge into an elongated porcelain columnwherein it meets a stream of water under sufficient pressure to break upthe fibrous mass and to disseminate large volumes of water throughoutthe mass. The drowned nitrocellulose issuing from the bottom of thecolumn has practically the .temperature of the diluting water, by whichwe mean that the temperature rise in the mass is at most only a fewNITROQELLULOSES a, 1931. Serial No. 561,002.

degrees. The drowned nitrocellulose is dischar ed into a storage tank,from which it is then pumped to the boiling or stabilizing vats, whereinit is safe to subject the highly diluted pulpous mass to boilingtemperatures, because of its low acidity. J" Contrary to such prior artpractice, we have discovered that it is advantageous .to drown freshlynitrated cellulose associated with the strong mixed nitrating acids, notre moved bydrainingor centrifuging, to drowning with warm water attemperatures appreciably above seasonable temperatures, but below about4:5" 0., as .a maximum. By this sort of practice, we have found thatimproved results are secured, principally in the clarity of thenitrocellulose solutions, when the nitrocellulose is put through theusual subsequent operations and is dissolved in the customarynitrocellulose solvents. The surprising thing is that no material lossin nitrocellulose is encountered so long as we avoid overstepping atemperature of about 45 C. in the diluting ordrowning water, but theloss in yield becomes increasingly more serious as the temperature israised above 45 C.,so much so, that at about C. or higher, the yield maydrop 10% or 20%, or even more. In other words, the yield ofnitrocellulose stays practically constantup to about 45 0;, whereabovethere is such a sharp drop in the yield that temperatures in theneighborhood of 45 C. represent a critical zone which should not beoverstepped.

While not limited thereto, our discovery is of vital significance inconnection with the nitration of refined wood pulps which, from thestandpoint of purity, alpha cellulose content, and othercharacteristics, should normally yield nitrocelluloses of a qualitycomparable to those realized from cotton linters. In the case of thebest refined wood pulps,

however, as is'also true with cotton linters, V

with the present invention, we are enabled to improve greatly theclarity and sparkle of the nitrocellulose solutions. Unless ourinvention is practised, one may not consistently secure nitrocellulosesolutions of the best clarity and sparkle. Thus when the customarydrowning procedure is followed, one may not run into the trouble ofhaziness in the nitrocellulose solution with several successive batches,but, sooner or later, onebatch may be appreciably off grade when it'isput into solution. This may be attributed to variationsin thecharacteristics of the cellulose fiber, in

acid composition, and in'temperature, time,-

and other factors of nitration. While we cannot with precision'accountfor the improved results which we secure, nevertheless our picture isthat haziness in the nitrocellulose solution is frequently-caused by thepresence in the nitrocellulose of nitrates of low solubility in thenitrocellulose solvents. These un desirable nitrates, which maybepresent in onlytraces, are evidently hydrolyzed and removed under theconditions of acidity and temperature prevailing during the diffusion ofthe warm diluting or drowningwater into the mass 'of nitrocellulose. Onthe other hand, the primary nitrocellulose product, which is completelysoluble in the nitrocellulose solvents, is evidently unafiected .undertheconditions prevailing during the drowning in warm water, providedthat a temperature of about C..is not exceeded.

Using comparatively thicksheets of refined wood pulp,.known as driersheets., the procedure may be about as follows. The sheets of wood pulpmay be conditioned for nitration in such a way as to yieldnitrocelluloses of. -sec0nd or lower solution viscosity; Theconditioning treatment may, for example, comprise two steps, viz., thestep of mercerization'with caustic soda solutions of 18% or greaterstrength at aboutroom temperature,followed by thestep of hydrolysis withsuitable acid, preferably dilute mineral acids, such ashydrochloric,sulphuric, or the like. Each of these major steps isfollowed by a washing operation in order to avoid'interference with thesucceeding step. The mercerized and hydrolyzed sheets are dried, and maythen be steeped for about two hours at about 30 C. in a mixed nitratingacid, whose composition may, for example, be 17.0%

water, 33.2% nitric acid, and 49.8% sulphuric acid. The nit-rating acidmaythen be drained from the sheets and be replaced by anothernitrocellulose of say, about .35 to 1, by weight, as this ensures atemperature throughout the drowned sheets safely below 45 C. Thedrowning water is drained from the sheets and is replaced by freshwater, in which the sheets .may be boiled, as through the directaddition of live steam, in order. to effect the usual stabilization. Thetwo-step nitrating operation hereinbefore given is one which has beenfound "to be; eminently suitable forthe nitration'ofthick sheets ofrefined wood pulp, asmore fully disclosed in application Serial No.451,995, filed May 13, 1930 by Milton 0. Schur. One may, however, useother suitable nitrating procedures, and then follow them up with a:drowning procedure with warm water in .accordance with our invene tion.'So,'.too,one'maygnitrate refined wood pulp in other forms, e. g'., inbulk or shredded condition, and. then subject the drained or centrifugednitrocellulose to our novel drowning step. In such latter instances, thedrowning may beefl'ected by playing a stream of the warm water undersuitable pressure onto thenitrated material so as to avoid localsufiiciently large volume of warm water but at a temperature not inexcess of about 45 -C. to ensure=practically thesame temperaturethroughout the drowned sheets.

In testimony whereof we have affixed our signatures. l

MILTON O. SCHUR. BENJAMIN G. HOOS.

mixed nitrating acid containing 18.7% water, v

20.5% nitric acid and 60.8% sulphuric acid,

' in which thesheets may be allowed to remain for about twohours at 30C. The second acid may be drained'from the nitrated sheets and the batchof acid-wet nitrocellulose then drowned in a large volume-of water atsay about 85C. Excellent resultshave been ob- =tained with a ratio ofdrowning water to

